In recent years, the reduction of carbon dioxide emissions has been sincerely desired in order to address global warming. The automotive industry has a growing expectation on the introduction of electric vehicles and hybrid electric vehicles for the reduction of carbon oxide emissions and has been intensively developing secondary batteries for motor drives, which become key to the practical application of these electric vehicles.
As the secondary batteries for motor drives, attentions are being given to lithium-ion secondary batteries having relatively high theoretical energy. The development of such lithium-ion secondary batteries is pursued rapidly at present. In the lithium-ion secondary battery, there are generally provided a positive electrode in which a positive electrode active material is applied to a positive electrode collector by a binder and a negative electrode in which a negative electrode active material is applied to a negative electrode collector by a binder. The positive electrode and the negative electrode are connected to each other via a liquid or solid electrolyte layer and are accommodated in a battery case. The lithium-ion secondary battery thus undergoes a charge/discharge reaction due to the absorption and desorption of lithium ions by the electrode active materials.
Alloy materials and carbon materials are used as the negative electrode active material of the lithium-ion secondary battery. The electrode active material however expands and contracts in response to the absorption and desorption of lithium ions during the charge/discharge reaction of the battery. For example, a carbon-based negative electrode active material such as graphite shows a volume change of about 10%; and an alloy-based negative electrode active material shows a volume change of nearly 200%.
When the active material shows a large volume expansion, the active material may be broken into fine pieces and separated from the collector during repeated charge/discharge cycles. Further, the electrode itself may be largely twisted and deformed when the collector sustains large stress in response to the volume change of the active material thin-film layer. There thus arises a problem that it is likely that the cycle characteristics of the battery will deteriorate as the contact between the active materials becomes reduced by the change of the electrode structure during the repeated charge/discharge cycles.
In order to solve such a problem, Patent Document 1 discloses a negative electrode for a non-aqueous electrolyte secondary battery, which has a foam metal as a collector and silicon supported as an active material on the foam metal. It is reported that it is possible by such an electrode configuration to prevent separation of the active material during charge/discharge cycles for improvements in charge/discharge cycle characteristics.